Grafted copolymers of a nitrogenous, heterocyclic monomer with a pendant ethylenically unsaturated moiety and a polyolefin have previously been proposed for use in lubricating oils as viscosity index improving (VII) agents and as dispersants for keeping the slurry of insoluble materials in the crankcase of an internal combustion engine in suspension during cold running of the engine. Among many graft polymers suggested for this use are ethylene-propylene polymers, grafted with N-vinylimidazole or other monomers. See U.S. Pat. No. 4,092,255.
Another material which has been suggested for use as both a viscosity index improver and a dispersant is a polymer containing N-vinyl pyrrolidone and an alkyl methacrylate. U.S. Pat. No. 4,146,489, col. 1, lines 51-62.
Previous dispersant/viscosity index improvers ("DVII's") have typically provided an ADT (asphaltene dispersancy test) value of from about 2 to about 4.
Grafted polyolefins for use as lubricating oil additives have previously been prepared by dissolving the selected polyolefin in a solvent (which may be a lubricating oil base stock), adding a peroxide as a free radical generator (also referred to in this specification as an initiator), holding the mixture at an elevated temperature to form active sites on the polyolefin, adding the graft monomer, and allowing the mixture to react at an elevated temperature for long enough to form the desired graft polyolefin. U.S. Pat. No. 4,092,255, col. 4, line 5.4, to col. 5, line 12. That patent states that the polydispersity of the grafted polyolefin is less than that of the starting polymer, which is said to improve its shearing stability. Col. 6, lines 17-20.
The prior art also suggests that the graft reaction to form a dispersant/VII grafted polyolefin can be controlled to avoid by-products by combining the polyolefin backbone, graft monomer, and initiator at a temperature below the initiation temperature of the initiator, then heating the mixture to above that initiation temperature to begin the reaction. U.S. Pat. No. 4,146,489, issued Mar. 27, 1979. Example 1 of the '479 patent suggests that the initiation temperature of di-t-butyl peroxide is between 160.degree. C. and 170.degree. C. Addition of the initiator in two stages is suggested in Example 4 of the same patent. A graft polymer containing 1-10% by weight, preferably 2-6% by weights most preferably about 3% by weight of the grafted monomer is taught. '489 patent, col. 3, lines 11-15.
One problem with prior grafted polyolefins is their limited shear stability--the ability to withstand extensive shearing, as in an internal combustion engine, without losing potency as viscosity-index improving additives. U.S. Pat. No. 4,146,489, col. 5, lines 48-58, states that: "During the grafting reaction, noticeable thickening takes place, and evaluation of the grafted polyolefin indicates that shear stability deteriorates during the grafting reaction. This very likely results from crosslinking that may occur as part of the reaction. Although it is possible to eliminate this crosslinking, the products so prepared generally are inferior dispersants. Hence, it appears to be inherent to some extent in the graft process of this invention that to obtain optimum dispersancy, some compromise in shear stability is necessary."
The '489 patent also points out that the shear stability of the grafted copolymer can be improved by mechanical or thermal degradation of the polymer to reduce the content of long-molecular-weight species which are readily broken down by such processes. However, it is not desirable to carry out the time-consuming and expensive processes necessary to mechanically or thermally degrade the polymer.
Another manner proposed for preventing side reactions in the grafting process, which are said to include cross-linking of polymer chains, homopolymerization of the graft monomer, or functionalization of the grafted polymer, is to run the reaction at a relatively high temperature, such as 190.degree. C. or more if a t-butyl peroxide catalyst is used. This expedient is also said to allow the proportion of grafted monomer in the resulting product to be increased, as well. U.S. Pat. No. 4,810,754, col. 2, lines 19-43.
A grafting reaction has been carried out, according to Example 1 of U.S. Pat. No. 4,810,754, by adding the initiator (di-t-butyl peroxide) and the graft monomer (2-vinyl pyridine--molecular weight 105.14) over a period of 45 minutes to an ethylene-propylene polyolefin reaction mixture maintained in a solvent mineral oil at 190.degree. C. The resulting grafted polymer was said to contain as much as 0.17% nitrogen by one analysis. This amounts to a 1.3% by weight vinylpyridine monomer grafting level.